The theft checking system uses a short wave band centered at, for example, 8.2 MHz, and the electromagnetic waves radiated from the transmission antenna are weak so that they are susceptible to communication signals, electromagnetic waves from fluorescent lamps or office automation appliances, and the like, which intrude as noises.
The influence of noises can be reduced relatively easily by increasing the electromagnetic wave transmission power, increasing the resonance circuit in size, or reducing the detection area. However, an increase in the electromagnetic wave transmission power is only permitted within a limit, and a size increase of the resonance circuit results in a correspondingly increased size of the tag, which is unfavorable.
A reduction in the detection area will by no means be acceptable to consumers when the demand for enlarged detection areas is presently increasing.
Accordingly, a conventional technology is proposed, as described in, for example, Japanese Patent Laid-Open No. Sho 63-126094, wherein two types of resonance circuits having different resonance frequencies are incorporated in a tag, and the electromagnetic waves to be transmitted are subjected to sweeping, and if the re-radiated electromagnetic waves are received from the two types of resonance circuits a total of four times during a period of sweep, validity is established and a detection signal is outputted.
The aforementioned technology achieves a high detection precision since it detects only re-radiated electromagnetic waves while ignoring eruptive noises. However, the tag doubles in size and thus offends the eye.
Moreover, if a signal indistinct from the resonance signal is detected four times within a period of sweep, the signal is mistaken as a resonance signal and false determination is made.